FRIDAY, July 29, 2016 (HealthDay News) — The United States is apparently experiencing its first local outbreak of the Zika virus, with four human infections reported in South Florida very likely caused by mosquito bites, federal health officials announced Friday.

One woman and three men in Miami-Dade and Broward counties have tested positive for Zika, and appear to have contracted the virus via mosquito bites, Florida Gov. Rick Scott said Friday during a media briefing.

In a statement, the federal Centers for Disease Control and Prevention confirmed that after months of warnings, Zika — which is primarily spread by mosquitoes and can cause devastating birth defects — appears to have finally reached America’s shores.

Until now, Zika infections have been confined to Latin America and the Caribbean, with Brazil the focal point of the outbreak.

“These [U.S.] cases are not unexpected. At CDC, we have been saying for months, based on experience with chikungunya and dengue — viruses spread by the same mosquito that spreads Zika — that individual cases and potentially small clusters of Zika are possible in the U.S.,” CDC Director Tom Frieden said during a Friday media briefing.

“As we have anticipated, Zika is now here,” Frieden said.

Miami-Dade County is one of the busiest entry points into the United States from countries where the Zika virus is circulating, making it one of the areas most at risk for a Zika outbreak, The New York Times reported.

The CDC has sent a medical epidemiologist to assist Florida officials in their ongoing investigation of the Zika infections, the agency said.

“We will continue to support Florida’s efforts to investigate and respond to Zika and will reassess the situation and our recommendations on a daily basis,” Frieden said.

While the Florida cases apparently mark the first local transmission of Zika in the continental United States, the CDC also announced Friday that Zika infections have skyrocketed to epidemic levels in Puerto Rico.

Positive tests for people with suspected Zika infection increased from 14 percent in February to 64 percent in June in Puerto Rico. As of July 7, Zika had been diagnosed in 5,582 people in Puerto Rico, including 672 pregnant women, the CDC said.

Zika infection poses significant risks to pregnant women, because it can cause the birth defect called microcephaly, which results in babies born with undersized heads and underdeveloped brains.

But the virus poses little threat to most other people, with about 80 percent of those infected never noticing any symptoms.

“Puerto Rico is in the midst of a Zika epidemic. The virus is silently and rapidly spreading in Puerto Rico,” said Dr. Lyle Peterson, incident manager for the CDC’s Zika response and director of the Division of Vector-Borne Diseases. “This could lead to hundreds of infants being born with microcephaly or other birth defects in the coming year.”

The CDC and other public health officials have said repeatedly they expect to see cases of local transmission of the Zika virus this summer in warm, humid southern states such as Florida, Louisiana and Texas. The virus is typically transmitted through the bite of Aedes mosquitoes.

But U.S. officials said they don’t expect to see a Zika epidemic in the United States similar to those in Latin America. The reason: better insect control as well as window screens and air conditioning that should help curtail any outbreaks.

Public health officials believe the Florida infections occurred in a several-block area just north of downtown Miami known as the Wynwood arts district where the patients live or work, Frieden said.

Florida officials became aware of the potential Zika cluster in mid-July, and an investigation of the area found a significant number of mosquitoes that can spread Zika, Frieden said.

“Florida is aggressively working to reduce mosquitoes in the area of the reported cases,” Frieden said. “They’ve been going door-to-door to reduce standing water. They’re spraying by both truck and backpack, both for adult mosquitoes and larval mosquitoes.”

Health officials also have been conducting community surveys to find other potentially Zika-infected people who live in or work in the area of infection, he added.

“We would not be surprised if additional individual cases are reported. In fact, there may well be more cases that we’re not aware of right now because most people infected with Zika don’t have symptoms,” Frieden said. “If, however, we were to see cases in this area with people infected after the mosquito-control efforts were undertaken, this would be of concern and warrant further advice and action.”

Scott said that area of Miami is currently the only part of the state being tested for potential local transmissions of Zika.

No Zika-infected mosquitoes have been found in the area. But, the CDC has long warned that it would have to rely on human infections to track Zika outbreaks in the United States. Finding a live mosquito carrying Zika is akin to “finding a needle in a haystack,” Frieden said during the media briefing.

Women living in that area of Miami who are pregnant or considering pregnancy are being urged to contact either their doctors or their county health department for Zika prevention kits.

However, at this time federal health officials have not recommended that pregnant women avoid travel to South Florida.

“We don’t currently see a situation where we would advise pregnant women not to travel there,” Frieden said. “However, if cases were to continue in that area even after the mosquito-control activities have been undertaken, that would be a very different situation.”

Aedes mosquitoes feed primarily on human blood, and tend to breed in small pools of water found in local neighborhoods.

The mosquitoes have a short travel range, and Florida officials are describing the infections as a “small case cluster” that do not indicate widespread transmission, the Times reported.

“They tend to bite locally, ZIP code by ZIP code,” Dr. Chris Curry, a clinician with Miami’s Jackson Memorial Hospital, told the newspaper.

Frieden concurred. “The Aedes aegypti mosquito does not travel more than 150 meters in its lifetime, and often quite a bit less than that,” he said.

Responding to the possibility of local transmission of the Zika virus in South Florida, the U.S. Food and Drug Administration said Thursday that it was asking all blood centers in Miami-Dade and Broward counties to stop collecting blood immediately. They can resume collections once the blood centers put in place testing for each unit of blood collected.

The FDA urged nearby counties to take similar precautions to safeguard blood supplies.

So far, the 1,658 Zika infections reported in the United States mainly have been linked to travel to countries with Zika outbreaks in Latin America or the Caribbean.

In addition to mosquitoes, the Zika virus can be transmitted through sex. The CDC has reported 14 cases of sexually transmitted infections. These infections are thought to have occurred because the patients’ partners had traveled to countries where Zika is circulating, the CDC said.

The Zika virus also has been linked to a rare paralyzing condition called Guillain-Barre syndrome.

The CDC advises pregnant women not to travel to an area where active Zika transmission is ongoing, and to use insect repellent and wear long pants and long-sleeved shirts if they are in those areas. Partners of pregnant women are advised to use a condom to guard against sexual transmission during pregnancy.

State officials along the Gulf Coast say a lack of funding has hampered their Zika response efforts. President Barack Obama has asked Congress to allocate $1.9 billion to combat the Zika threat, but federal lawmakers have yet to agree on a spending package.

More information

The U.S. Centers for Disease Control and Prevention provides more information on mosquito-borne diseases.

This Q&A will tell you what you need to know about Zika.

To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.